1. Field of the Invention
The invention described hereinbelow relates to the field of semiconductor processing, and more specifically to a device and method for calibrating an optical pyrometer to an external blackbody reference point.
2. Description of the Relevant Art
Temperature is a critical variable in many semiconductor processing operations. Thermal annealing of wafers is an example of a high temperature semiconductor processing operation for which precision temperature measurements are important. A thermal anneal step is required after ion implantation in order to diffuse and activate the implanted ions and repair possible implant damage to the crystal structure of the semiconductor substrate. An anneal can occur in furnace or a more modern rapid thermal anneal ("RTA") chamber. A typical RTA process is performed at 420-1200.degree. C. and lasts anywhere from a few seconds to a few minutes.
One of the major problems with RTA's is the difficulty in establishing the temperature of the RTA, and more specifically, the temperature at the surface of the wafer. Accurate temperature measurement is the most important and limiting factor in current RTA systems. In order to avoid contamination of wafers being processed inside the RTA chamber, preferred temperature measurements are made without physically contacting the wafers or the processing environment inside the RTA.
Radiation pyrometers are a general class of temperature measurement devices which provide the benefit of not having to touch the material being measured. Most pyrometers work by measuring radiation from the object whose temperature is to be measured. Optical pyrometers, which are often used in conjunction with RTAs, measure the temperature of incandescent bodies, such as heated wafers, by comparing them visually with a calibrated incandescent filament that can be adjusted in temperature. By running a current through a filament in the optical pyrometer, the filament begins to emit light, i.e. it becomes incandescent. The light emitted by the filament is characteristic of the temperature of the filament. An optical port built into the wall of the RTA chamber allows light from the heated wafer to be received by the optical pyrometer and visually compared to the color of the calibrated incandescent filament. The temperature of the heated wafer is established when the filament disappears against the background light of the incandescing wafer.
One of the problems with using optical pyrometers to measure the temperature of wafers in RTA chambers is the unreliability of optical pyrometers over long periods of time. As the number of measurements increases the accuracy of the optical pyrometer decreases. Deviations on the order of 10 degrees centigrade are possible in standard RTA processes.
One common method for calibrating optical pyrometers used in RTA processes involves placing a test wafer into the RTA chamber. A suitable test wafer in most cases will be a wafer with a reference thermocouple attached. The temperature value measured with the optical pyrometer is compared to the temperature value measured with the thermocouple. Reliable thermocouples are readily available in the market, and services such as the National Institute of Standards and Technology and the National Bureau of Standards provide calibration of thermocouples. Thus, thermocouples generally provide reliable reference temperatures.
However, in the case of RTA processes a number of problems are associated with this method of calibration. First, placement of the thermocouple into the RTA chamber creates a potential source of contamination which may result in an inaccurate temperature reading. A second problem is an increase in manufacturing costs resulting from downtime on the RTA when the test wafer is inserted and the testing process is carried out. As to the first problem, one possible source of contamination arises because the thermocouple must be electrically connected to a signal processing device outside of the RTA chamber. Another source of contamination is the solder used to attach the thermocouple to the wafer. At least one invention has contemplated a method of calibrating an optical pyrometer that circumvents the potential contamination problems that may accompany using a reference thermocouple fixed on a wafer.
In Dilhac et al. (U.S. Pat. No. 5,553,939), a test wafer with a reference region having an electromagnetic wave reflection discontinuity at a known temperature value is used. The advantage of this method is that the reflection discontinuity region does not present a potential source of contamination. Additionally, the reference discontinuity region can be put on the surface of the wafer opposite to where the optical pyrometer will be aimed. In this way, the light reflected from the reference region will not interfere with the pyrometer reading. While Dilhac et al. presents one way to overcome the contamination problem, it does not solve the second problem of higher costs due to RTA downtime.
Another conventional method of calibrating optical pyrometers is with blackbody calibration furnaces. Technically a blackbody is a surface that absorbs as much radiation as it emits, and the nature of the emitted radiation can be fully characterized by the temperature of the surface. All blackbodies have the same relationship between intensity or spectral radiance (which is what the optical pyrometer measures) and the wavelength of the light emitted by the blackbody. Because the emitted radiation does not depend on the size of the blackbody, relatively small but effective blackbodies are obtainable. The limiting factor on size becomes the dimensions of the heating chamber which is required to maintain the temperture at the surface of the blackbody. While blackbody references are conventional, either the optical pyrometer or the blackbody reference must be physically moved to allow calibration. In the case of an optical pyrometer fixed to an RTA chamber these options are not available. Thus, in addition to avoiding modification of the RTA process, it would be advantageous to provide a means for calibrating an optical pyrometer that does not require physical movement of either the pyrometer or the reference.